Adrenergic neurons play a major role in the innervation of heart, blood vessel and smooth muscle tissue. Compounds capable of interacting with adrenoceptor sites within adrenergic nerves can initiate a variety of physiological responses, including vasoconstfiction, vasodilation, and increased or decreased heart rate (chronotropic), contractility (inotropic) and metabolic activity. In the past, various adrenergic compounds have been employed to affect these and other physiological responses. However, many adrenergic compounds do not possess significant selectivity to enable desirable interactions with adrenergic receptor sites. That is, these adrenergic compounds do not demonstrate a high degree of specificity for differing receptor types within adrenergic neurons in order to obtain a desired physiological response separate from other possible, and perhaps less desirable, responses of the system.
Benign prostatic hyperplasia (BPH) is a condition which develops in middle-aged and elderly males and refers to the benign overgrowth of the stromal and epithelial elements of the prostate associated with aging. Symptoms of BPH include increased frequency of urination, nocturia, a weak urine stream and hesitancy or delay in starting the urine flow. Chrome consequences of BPH can include hypertrophy of bladder smooth muscle, a decompensated bladder and an increased incidence of urinary tract infection.
Typically, BPH begins at an age in the mid-fifties and is the most common cause of urinary tract problems of men of this age. BPH is apparently rare in men prior to age 40, but at age 60, approximately 50% of men have histological evidence of BPH. The prevalence of BPH continues to increase with age until, at age 80, approximately 80% of men have pathological evidence of BPH.
Although prostatic hyperplasia is a common finding in older men, the presence of urinary symptoms is the essential feature that distinguishes simple anatomic enlargement of the prostate from prostatism, which is the clinical syndrome whereby the patient experiences significant obstruction of urinary flow. It is not uncommon in older men to have a palpably enlarged prostate without showing the symptoms of prostatism. From the patient's perspective, however, the incidence and progression of urinary symptoms are more important than the mere presence of an enlarged prostate.
The discovery in the 1970's (M. Caine, et al., Brit. J. Urol., 47:193-202 (1975)) of large numbers of alpha-adrenergic receptors in the smooth muscle of the prostatic capsule and bladder neck led to the conclusion that there is both a static and a dynamic component to bladder outlet obstruction associated with BPH. The static component derives from the progressive hyperplasia of the prostate with aging, leading to urethral narrowing which causes symptoms of urinary obstruction. Superimposed on this essentially mechanical problem is the variable degree of smooth muscle contraction controlled by the sympatheic nervous system and which is affected by by factors such as stress, cold and sympathomimetic drugs. It is this dynamic component which explains the often rapid fluctuations in symptoms observed in patients with prostatism.
The currently most effective treatment for BPH is the surgical procedure of transurethral resection of the prostate (TURP) Since it removes the obstructing tissue (C. Chapple, Br. Med. Journal 304:1198-1199 (1992)) it is a treatment which is directed to the static and dynamic components of BPH. However, this surgical treatment is associated with rates of mortality (1%) and adverse event (incontinence 2-4%, infection 5-10%, and impotence 5-10%). A non-invasive alternative treatment would thus be highly desirable.
The incidental clinical observation that urinary incontinence developed in women during antihypertensive treatment with prazosin (T. Thien, K. P. Delacre, F. M. J. Debruyne, R. A. P. Koene, Br. Med. Journal, 622-623 (1978)) and the experimental work of Caine (op cit.) contributed to the recognition of the potential role of selective .alpha.-1 adrenoceptor blockade in diseases of the lower urinary tract. Subsequent studies by several groups have documented the functional role of .alpha.-1 adrenoceptors relative to .alpha.-2 adrenoceptors in the stromal compartment of the prostate, thereby providing a putative molecular basis for the use of specific .alpha.-1 adrenoceptor blockers in the non-surgical management of BPH (C. R. Chapple, M. L. Aubry, S. James, M. Greengrass, G. Burnstock, R. T. Turner-Warwick, Br. J. Urol. 63: 487-496 (1989)). Clinical efficacy of .alpha.-1 antagonists in BPH has been demonstrated with several non-selective .alpha.-1 blockers, including terazosin (Hytrin.TM.), prazosin, and doxazosin. Treatment periods as short as two to four weeks with .alpha.-1 adrenoceptor blockers have shown objective improvements in the mean and maximum urinary flow rates (14-96% ) with subjective improvements in patients' symptom scores (R. A. Janknegt, C. R. Chapple, Eur. Urol. 24: 319-326 (1993)). Longer term studies with terazosin, indoramin, prazosin, and doxazosin have similarly demonstrated significant improvements in urinary flow rates and subjective symptom scores (R. A. Janknegt, op. cit., H. Lepor, G. Knapp-Maloney, J. Urol. 145: 263A (1991), W. Chow, D. Hahn, D. Sandhu, Br. J. Urol. 65: 36-38 (1990) and C. R. Chapple, T. J. Christmas, E. J. G. Milroy, Urol. Int. 45: 47-55 (1990)). However, these agents possess similar dose limiting side effects: hypotension, dizziness, and muscle fatigue. Them thus exists a need for a "uroselective" .alpha.-1 antagonist with reduced side effect liabilities.